multipath.conf

package: multipath-tools

title: multipath.conf(5)

summary: multipath daemon configuration file.

text:
# multipath.conf(5)

## NAME
multipath.conf - multipath daemon configuration file.

## DESCRIPTION
/etc/multipath.conf is the configuration file for the multipath daemon. It is used to overwrite the built-in configuration table of multipathd. Any line whose first non-white-space character is a '#' is considered a comment line. Empty lines are ignored.

Currently used multipathd configuration can be displayed with the multipath -t or multipathd show config command.

## SYNTAX
The configuration file contains entries of the form:

```
<section> {
```

```
<attribute> <value>
...
<subsection> {
```

```
<attribute> <value>
...
```

```
}
```

```
}
```

Each section contains one or more attributes or subsections. The recognized keywords for attributes or subsections depend on the section in which they occur.

<attribute> and <value> must be on a single line. <attribute> is one of the keywords listed in this man page. <value> is either a simple word (containing no whitespace and none of the characters '"', '#', and '!') or one string enclosed in double quotes ("..."). Outside a quoted string, text starting with '#', and '!' is regarded as a comment and ignored until the end of the line. Inside a quoted string, '#' and '!' are normal characters, and whitespace is preserved. To represent a double quote character inside a double quoted string, use two consecutive double quotes ('""'). Thus '2.5" SSD' can be written as "2.5"" SSD".

Opening braces ('{') must follow the (sub)section name on the same line. Closing braces ('}') that mark the end of a (sub)section must be the only non-whitespace character on the line. Whitespace is ignored except inside double quotes, thus the indentation shown in the above example is helpful for human readers but not mandatory.

Note on regular expressions: The multipath.conf syntax allows many attribute values to be specified as POSIX Extended Regular Expressions (see regex(7)). These regular expressions are case sensitive and not anchored, thus the expression "bar" matches "barbie", "rhabarber", and "wunderbar", but not "Barbie". To avoid unwanted substring matches, standard regular expression syntax using the special characters "^" and "$" can be used.

The following section keywords are recognized:

defaults

This section defines default values for attributes which are used whenever no values are given in the appropriate device or multipath sections.

blacklist

This section defines which devices should be excluded from the multipath topology discovery.

blacklist_exceptions

This section defines which devices should be included in the multipath topology discovery, despite being listed in the blacklist section.

multipaths

This section defines the multipath topologies. They are indexed by a World Wide Identifier(WWID). For details on the WWID generation see section WWID generation below. Attributes set in this section take precedence over all others.

devices

This section defines the device-specific settings. Devices are identified by vendor, product, and revision.

overrides

This section defines values for attributes that should override the device-specific settings for all devices.

## defaults section
The defaults section recognizes the following keywords:

verbosity

Default verbosity. Higher values increase the verbosity level. Valid levels are between 0 and 6.

The default is: 2

polling_interval

Interval between two path checks in seconds. For properly functioning paths, the interval between checks will gradually increase to max_polling_interval. This value will be overridden by the WatchdogSec setting in the multipathd.service definition if systemd is used.

The default is: 5

max_polling_interval

Maximal interval between two path checks in seconds.

The default is: 4 * polling_interval

reassign_maps

Enable reassigning of device-mapper maps. With this option multipathd will remap existing device-mapper maps to always point to multipath device, not the underlying block devices. Possible values are yes and no.

The default is: no

multipath_dir

This option is not supported any more. The value is ignored.

path_selector

The default path selector algorithm to use; they are offered by the kernel multipath target:

round-robin 0

Loop through every path in the path group, sending the same amount of I/O to each. Some aspects of behavior can be controlled with the attributes: rr_min_io, rr_min_io_rq and rr_weight.

queue-length 0

(Since 2.6.31 kernel) Choose the path for the next bunch of I/O based on the amount of outstanding I/O to the path.

service-time 0

(Since 2.6.31 kernel) Choose the path for the next bunch of I/O based on the amount of outstanding I/O to the path and its relative throughput.

historical-service-time 0

(Since 5.8 kernel) Choose the path for the next bunch of I/O based on the estimation of future service time based on the history of previous I/O submitted to each path.

io-affinity 0

(Since 5.11 kernel) Choose the path for the next bunch of I/O based on a CPU to path mapping the user passes in and what CPU we are executing on.

The default is: service-time 0

path_grouping_policy

The default path grouping policy to apply to unspecified multipaths. Possible values are:

failover

One path per priority group.

multibus

All paths in one priority group.

group_by_serial

One priority group per serial number.

group_by_prio

One priority group per priority value. Priorities are determined by callout programs specified as a global, per-controller or per-multipath option in the configuration file.

group_by_node_name

One priority group per target node name. Target node names are fetched in /sys/class/fc_transport/target*/node_name.

The default is: failover

uid_attrs

Setting this option activates merging uevents by WWID, which may improve uevent processing efficiency. Moreover, it's an alternative method to configure the udev properties to use for determining unique path identifiers (WWIDs).

The value of this option is a space separated list of records like "type:ATTR", where type is matched against the beginning of the device node name (e.g. sd:ATTR matches sda), and ATTR is the name of the udev property to use for matching devices.

If this option is configured and matches the device node name of a device, it overrides any other configured methods for determining the WWID for this device.

This option cannot be changed during runtime with the multipathd reconfigure command.

The default is: <unset>. To enable uevent merging, set it e.g. to "sd:ID_SERIAL dasd:ID_UID nvme:ID_WWN".

uid_attribute

The udev attribute providing a unique path identifier (WWID). If uid_attribute is set to the empty string, WWID determination is done using the sysfs method rather then using udev (not recommended in production; see WWID generation below).

The default is: ID_SERIAL, for SCSI devices

The default is: ID_UID, for DASD devices

The default is: ID_WWN, for NVMe devices

getuid_callout

This option is not supported any more. The value is ignored.

prio

The name of the path priority routine. The specified routine should return a numeric value specifying the relative priority of this path. Higher number have a higher priority. "none" is a valid value. Currently the following path priority routines are implemented:

const

Return a constant priority of 1.

sysfs

Use the sysfs attributes access_state and preferred_path to generate the path priority. This prioritizer accepts the optional prio_arg exclusive_pref_bit.

emc

(Hardware-dependent) Generate the path priority for DGC class arrays as CLARiiON CX/AX and EMC VNX families with Failover Mode 1 (Passive Not Ready(PNR)).

alua

(Hardware-dependent) Generate the path priority based on the SCSI-3 ALUA settings. This prioritizer accepts the optional prio_arg exclusive_pref_bit.

ontap

(Hardware-dependent) Generate the path priority for NetApp ONTAP class, and rebranded arrays.

rdac

(Hardware-dependent) Generate the path priority for LSI/Engenio/NetApp RDAC class as NetApp SANtricity E/EF Series and rebranded arrays, with "Linux DM-MP (Kernel 3.9 or earlier)" option.

hp_sw

(Hardware-dependent) Generate the path priority for HP/COMPAQ/DEC HSG80 and MSA/HSV arrays with Active/Standby mode exclusively.

hds

(Hardware-dependent) Generate the path priority for Hitachi AMS families of arrays other than AMS 2000.

random

Generate a random priority between 1 and 10.

weightedpath

Generate the path priority based on the regular expression and the priority provided as argument. Requires prio_args keyword.

path_latency

Generate the path priority based on a latency algorithm. Requires prio_args keyword.

ana

(Hardware-dependent) Generate the path priority based on the NVMe ANA settings.

datacore

(Hardware-dependent) Generate the path priority for some DataCore storage arrays. Requires prio_args keyword.

iet

(iSCSI only) Generate path priority for iSCSI targets based on IP address. Requires prio_args keyword.

The default depends on the detect_prio setting: If detect_prio is yes (default), the default priority algorithm is sysfs (except for NetAPP E-Series, where it is alua). If detect_prio is no, the default priority algorithm is const.

prio_args

Arguments to pass to to the prio function. This only applies to certain prioritizers:

weighted

Needs a value of the form "<hbtl|devname|serial|wwn> <regex1> <prio1> <regex2> <prio2> ..."

hbtl

Regex can be of SCSI H:B:T:L format. For example: 1:0:.:. , *:0:0:.

devname

Regex can be of device name format. For example: sda , sd.e

serial

Regex can be of serial number format. For example: .*J1FR.*324 . The serial can be looked up through sysfs or by running multipathd show paths format "%z". For example: 0395J1FR904324

wwn

Regex can be of the form "host_wwnn:host_wwpn:target_wwnn:target_wwpn" these values can be looked up through sysfs or by running multipathd show paths format "%N:%R:%n:%r". For example: 0x200100e08ba0aea0:0x210100e08ba0aea0:.*:.* , .*:.*:iqn.2009-10.com.redhat.msp.lab.ask-06:.*

path_latency

Needs a value of the form "io_num=<20> base_num=<10>"

io_num

The number of read IOs sent to the current path continuously, used to calculate the average path latency. Valid Values: Integer, [2, 200].

base_num

The base number value of logarithmic scale, used to partition different priority ranks. Valid Values: Integer, [2, 10]. And Max average latency value is 100s, min average latency value is 1us. For example: If base_num=10, the paths will be grouped in priority groups with path latency <=1us, (1us, 10us], (10us, 100us], (100us, 1ms], (1ms, 10ms], (10ms, 100ms], (100ms, 1s], (1s, 10s], (10s, 100s], >100s.

alua

If exclusive_pref_bit is set, paths with the preferred path bit set will always be in their own path group.

sysfs

If exclusive_pref_bit is set, paths with the preferred path bit set will always be in their own path group.

datacore

preferredsds

(Mandatory) The preferred "SDS name".

timeout

(Optional) The timeout for the INQUIRY, in ms.

iet

preferredip=...

(Mandatory) Th preferred IP address, in dotted decimal notation, for iSCSI targets.

The default is: <unset>

features

Specify any device-mapper features to be used. Syntax is num list where num is the number, between 0 and 8, of features in list. Possible values for the feature list are:

queue_if_no_path

(Deprecated, superseded by no_path_retry) Queue I/O if no path is active. Identical to the no_path_retry with queue value. If both this feature and no_path_retry are set, the latter value takes precedence. See KNOWN ISSUES.

pg_init_retries <times>

(Since kernel 2.6.24) Number of times to retry pg_init, it must be between 1 and 50.

pg_init_delay_msecs <msecs>

(Since kernel 2.6.38) Number of msecs before pg_init retry, it must be between 0 and 60000.

queue_mode <mode>

(Since kernel 4.8) Select the queueing mode per multipath device. <mode> can be bio, rq or mq, which corresponds to bio-based, request-based, and block-multiqueue (blk-mq) request-based, respectively. Before kernel 4.20 The default depends on the kernel parameter dm_mod.use_blk_mq. It is mq if the latter is set, and rq otherwise. Since kernel 4.20, rq and mq both correspond to block-multiqueue. Once a multipath device has been created, its queue_mode cannot be changed. nvme:tcp paths are only supported in multipath devices with queue_mode set to bio. multipath will automatically set this when creating a device with nvme:tcp paths.

The default is: <unset>

path_checker

The default method used to determine the path's state. The synchronous checkers (all except tur and directio) will cause multipathd to pause most activity, waiting up to checker_timeout seconds for the path to respond. The asynchronous checkers (tur and directio) will not pause multipathd. Instead, multipathd will check for a response once per second, until checker_timeout seconds have elapsed. Possible values are:

readsector0

(Deprecated) Read the first sector of the device. This checker is being deprecated, please use tur instead.

tur

Issue a TEST UNIT READY command to the device.

emc_clariion

(Hardware-dependent) Query the DGC/EMC specific EVPD page 0xC0 to determine the path state for CLARiiON CX/AX and EMC VNX and Unity arrays families.

hp_sw

(Hardware-dependent) Check the path state for HP/COMPAQ/DEC HSG80 and MSA/HSV arrays with Active/Standby mode exclusively.

rdac

(Hardware-dependent) Check the path state for LSI/Engenio/NetApp RDAC class as NetApp SANtricity E/EF Series, and rebranded arrays.

directio

Read the first sector with direct I/O. This checker could cause spurious path failures under high load. Increasing checker_timeout can help with this.

cciss_tur

(Hardware-dependent) Check the path state for HP/COMPAQ Smart Array(CCISS) controllers.

none

Do not check the device, fallback to use the values retrieved from sysfs

The default is: tur

alias_prefix

The user_friendly_names prefix.

The default is: mpath

failback

Tell multipathd how to manage path group failback. To select immediate or a value, it's mandatory that the device has support for a working prioritizer.

immediate

Immediately failback to the highest priority pathgroup that contains active paths.

manual

Do not perform automatic failback.

followover

Used to deal with multiple computers accessing the same Active/Passive storage devices. Only perform automatic failback when the first path of a pathgroup becomes active. This keeps a cluster node from automatically failing back when another node requested the failover.

values > 0

Deferred failback (time to defer in seconds).

The default is: manual

rr_min_io

Number of I/O requests to route to a path before switching to the next in the same path group. This is only for Block I/O(BIO) based multipath and only apply to round-robin path_selector.

The default is: 1000

rr_min_io_rq

Number of I/O requests to route to a path before switching to the next in the same path group. This is only for Request based multipath and only apply to round-robin path_selector.

The default is: 1

max_fds

Specify the maximum number of file descriptors that can be opened by multipath and multipathd. This is equivalent to ulimit -n. A value of max will set this to the system limit from /proc/sys/fs/nr_open. If this is not set, the maximum number of open fds is taken from the calling process. It is usually 1024. To be safe, this should be set to the maximum number of paths plus 32, if that number is greater than 1024.

The default is: max

rr_weight

If set to priorities the multipath configurator will assign path weights as "path prio * rr_min_io". Possible values are priorities or uniform . Only apply to round-robin path_selector.

The default is: uniform

no_path_retry

Specify what to do when all paths are down. Possible values are:

value > 0

Number of retries until disable I/O queueing.

fail

For immediate failure (no I/O queueing).

queue

For never stop I/O queueing, similar to queue_if_no_path. See KNOWN ISSUES.

The default is: fail

queue_without_daemon

If set to no , when multipathd stops, queueing will be turned off for all devices. This is useful for devices that set no_path_retry. If a machine is shut down while all paths to a device are down, it is possible to hang waiting for I/O to return from the device after multipathd has been stopped. Without multipathd running, access to the paths cannot be restored, and the kernel cannot be told to stop queueing I/O. Setting queue_without_daemon to no , avoids this problem.

The default is: no

checker_timeout

Specify the timeout to use for path checkers and prioritizers, in seconds. Only prioritizers that issue scsi commands use checker_timeout. If a path does not respond to the checker command after checker_timeout seconds have elapsed, it is considered down.

The default is: in /sys/block/<dev>/device/timeout

allow_usb_devices

If set to no , all USB devices will be skipped during path discovery. If you intend to use multipath on USB attached devices, set this to yes.

The default is: no

flush_on_last_del

If set to yes , multipathd will disable queueing when the last path to a device has been deleted.

The default is: no

user_friendly_names

If set to yes , using the bindings file /etc/multipath/bindings to assign a persistent and unique alias to the multipath, in the form of mpath<n>. If set to no use the WWID as the alias. In either case this be will be overridden by any specific aliases in the multipaths section.

The default is: no

fast_io_fail_tmo

Specify the number of seconds the SCSI layer will wait after a problem has been detected on a FC remote port before failing I/O to devices on that remote port. This should be smaller than dev_loss_tmo. Setting this to off will disable the timeout.

The default is: 5

dev_loss_tmo

Specify the number of seconds the SCSI layer will wait after a problem has been detected on a FC remote port before removing it from the system. This can be set to "infinity" which sets it to the max value of 2147483647 seconds, or 68 years. It will be automatically adjusted to the overall retry interval no_path_retry * polling_interval if a number of retries is given with no_path_retry and the overall retry interval is longer than the specified dev_loss_tmo value. The Linux kernel will cap this value to 600 if fast_io_fail_tmo is not set. See KNOWN ISSUES.

The default is: 600

eh_deadline

Specify the maximum number of seconds the SCSI layer will spend doing error handling when scsi devices fail. After this timeout the scsi layer will perform a full HBA reset. Setting this may be necessary in cases where the rport is never lost, so fast_io_fail_tmo and dev_loss_tmo will never trigger, but (frequently do to load) scsi commands still hang. Note: when the scsi error handler performs the HBA reset, all target paths on that HBA will be affected. eh_deadline should only be set in cases where all targets on the affected HBAs are multipathed.

The default is: <unset>

bindings_file

This option is deprecated, and will be removed in a future release. The full pathname of the binding file to be used when the user_friendly_names option is set.

The default is: /etc/multipath/bindings

wwids_file

This option is deprecated, and will be removed in a future release. The full pathname of the WWIDs file, which is used by multipath to keep track of the WWIDs for LUNs it has created multipath devices on in the past.

The default is: /etc/multipath/wwids

prkeys_file

This option is deprecated, and will be removed in a future release. The full pathname of the prkeys file, which is used by multipathd to keep track of the persistent reservation key used for a specific WWID, when reservation_key is set to file.

The default is: /etc/multipath/prkeys

log_checker_err

If set to once , multipathd logs the first path checker error at logging level 2. Any later errors are logged at level 3 until the device is restored. If set to always , multipathd always logs the path checker error at logging level 2.

The default is: always

reservation_key

This is the service action reservation key used by mpathpersist. It must be set for all multipath devices using persistent reservations, and it must be the same as the RESERVATION KEY field of the PERSISTENT RESERVE OUT parameter list which contains an 8-byte value provided by the application client to the device server to identify the I_T nexus. If the --param-aptpl option is used when registering the key with mpathpersist, :aptpl must be appended to the end of the reservation key.

Alternatively, this can be set to file, which will store the RESERVATION KEY registered by mpathpersist in the prkeys_file. multipathd will then use this key to register additional paths as they appear. When the registration is removed, the RESERVATION KEY is removed from the prkeys_file. The prkeys file will automatically keep track of whether the key was registered with --param-aptpl.

The default is: <unset>

all_tg_pt

Set the 'all targets ports' flag when registering keys with mpathpersist. Some arrays automatically set and clear registration keys on all target ports from a host, instead of per target port per host. The ALL_TG_PT flag must be set to successfully use mpathpersist on these arrays. Setting this option is identical to calling mpathpersist with --param-alltgpt

The default is: no

retain_attached_hw_handler

(Obsolete for kernels >= 4.3) If set to yes and the SCSI layer has already attached a hardware_handler to the device, multipath will not force the device to use the hardware_handler specified by multipath.conf. If the SCSI layer has not attached a hardware handler, multipath will continue to use its configured hardware handler.

The default is: yes

Important Note: Linux kernel 4.3 or newer always behaves as if "retain_attached_hw_handler yes" was set.

detect_prio

If set to yes , multipath will try to detect if the device supports SCSI-3 ALUA. If so, the device will automatically use the sysfs prioritizer if the required sysf attributes access_state and preferred_path are supported, or the alua prioritizer if not. If set to no , the prioritizer will be selected as usual.

The default is: yes

detect_checker

if set to yes , multipath will try to detect if the device supports SCSI-3 ALUA. If so, the device will automatically use the tur checker. If set to no , the checker will be selected as usual.

The default is: yes

force_sync

If set to yes , multipathd will call the path checkers in sync mode only. This means that only one checker will run at a time. This is useful in the case where many multipathd checkers running in parallel causes significant CPU pressure.

The default is: no

strict_timing

If set to yes , multipathd will start a new path checker loop after exactly one second, so that each path check will occur at exactly polling_interval seconds. On busy systems path checks might take longer than one second; here the missing ticks will be accounted for on the next round. A warning will be printed if path checks take longer than polling_interval seconds.

The default is: no

deferred_remove

If set to yes , multipathd will do a deferred remove instead of a regular remove when the last path device has been deleted. This means that if the multipath device is still in use, it will be freed when the last user closes it. If path is added to the multipath device before the last user closes it, the deferred remove will be canceled.

The default is: no

partition_delimiter

This parameter controls how multipath chooses the names of partition devices of multipath maps if a multipath map is renamed (e.g. if a map alias is added or changed). If this parameter is set to a string other than "/UNSET/" (even the empty string), multipath inserts that string between device name and partition number to construct the partition device name. Otherwise (i.e. if this parameter is unset or has the value "/UNSET/"), the behavior depends on the map name: if it ends in a digit, a "p" is inserted between name and partition number; otherwise, the partition number is simply appended. Distributions may use a non-null default value for this option; in this case, the user must set it to "/UNSET/" to obtain the original <unset> behavior. Use multipath -T to check the current settings.

The default is: <unset>

config_dir

This option is not supported any more. The value is ignored.

san_path_err_threshold

If set to a value greater than 0, multipathd will watch paths and check how many times a path has been failed due to errors.If the number of failures on a particular path is greater than the san_path_err_threshold, then the path will not reinstate till san_path_err_recovery_time. These path failures should occur within a san_path_err_forget_rate checks, if not we will consider the path is good enough to reinstantate. See "Shaky paths detection" below.

The default is: no

san_path_err_forget_rate

If set to a value greater than 0, multipathd will check whether the path failures has exceeded the san_path_err_threshold within this many checks i.e san_path_err_forget_rate . If so we will not reinstate the path till san_path_err_recovery_time. See "Shaky paths detection" below.

The default is: no

san_path_err_recovery_time

If set to a value greater than 0, multipathd will make sure that when path failures has exceeded the san_path_err_threshold within san_path_err_forget_rate then the path will be placed in failed state for san_path_err_recovery_time duration.Once san_path_err_recovery_time has timeout we will reinstate the failed path . san_path_err_recovery_time value should be in secs. See "Shaky paths detection" below.

The default is: no

marginal_path_double_failed_time

One of the four parameters of supporting path check based on accounting IO error such as intermittent error. When a path failed event occurs twice in marginal_path_double_failed_time seconds due to an IO error and all the other three parameters are set, multipathd will fail the path and enqueue this path into a queue of which members are sent a couple of continuous direct reading asynchronous IOs at a fixed sample rate of 10HZ to start IO error accounting process. See "Shaky paths detection" below.

The default is: no

marginal_path_err_sample_time

One of the four parameters of supporting path check based on accounting IO error such as intermittent error. If it is set to a value no less than 120, when a path fail event occurs twice in marginal_path_double_failed_time second due to an IO error, multipathd will fail the path and enqueue this path into a queue of which members are sent a couple of continuous direct reading asynchronous IOs at a fixed sample rate of 10HZ to start the IO accounting process for the path will last for marginal_path_err_sample_time. If the rate of IO error on a particular path is greater than the marginal_path_err_rate_threshold, then the path will not reinstate for marginal_path_err_recheck_gap_time seconds unless there is only one active path. After marginal_path_err_recheck_gap_time expires, the path will be requeued for rechecking. If checking result is good enough, the path will be reinstated. See "Shaky paths detection" below.

The default is: no

marginal_path_err_rate_threshold

The error rate threshold as a permillage (1/1000). One of the four parameters of supporting path check based on accounting IO error such as intermittent error. Refer to marginal_path_err_sample_time. If the rate of IO errors on a particular path is greater than this parameter, then the path will not reinstate for marginal_path_err_recheck_gap_time seconds unless there is only one active path. See "Shaky paths detection" below.

The default is: no

marginal_path_err_recheck_gap_time

One of the four parameters of supporting path check based on accounting IO error such as intermittent error. Refer to marginal_path_err_sample_time. If this parameter is set to a positive value, the failed path of which the IO error rate is larger than marginal_path_err_rate_threshold will be kept in failed state for marginal_path_err_recheck_gap_time seconds. When marginal_path_err_recheck_gap_time seconds expires, the path will be requeued for checking. If checking result is good enough, the path will be reinstated, or else it will keep failed. See "Shaky paths detection" below.

The default is: no

delay_watch_checks

This option is deprecated, and mapped to san_path_err_forget_rate. If this is set to a value greater than 0 and no san_path_err options are set, san_path_err_forget_rate will be set to the value of delay_watch_checks and san_path_err_threshold will be set to 1. See the san_path_err_forget_rate and san_path_err_threshold options, and "Shaky paths detection" below for more information.

The default is: no

delay_wait_checks

This option is deprecated, and mapped to san_path_err_recovery_time. If this is set to a value greater than 0 and no san_path_err options are set, san_path_err_recovery_time will be set to the value of delay_wait_checks times max_polling_interval. This will give approximately the same wait time as delay_wait_checks previously did. Also, san_path_err_threshold will be set to 1. See the san_path_err_recovery_time and san_path_err_threshold options, and "Shaky paths detection" below for more information.

The default is: no

marginal_pathgroups

If set to off, the delay_*_checks, marginal_path_*, and san_path_err_* options will keep marginal, or "shaky", paths from being reinstated until they have been monitored for some time. This can cause situations where all non-marginal paths are down, and no paths are usable until multipathd detects this and reinstates a marginal path. If the multipath device is not configured to queue IO in this case, it can cause IO errors to occur, even though there are marginal paths available. However, if this option is set to on, when one of the marginal path detecting methods determines that a path is marginal, it will be reinstated and placed in a separate pathgroup that will only be used after all the non-marginal pathgroups have been tried first. This prevents the possibility of IO errors occurring while marginal paths are still usable. After the path has been monitored for the configured time, and is declared healthy, it will be returned to its normal pathgroup. If this option is set to fpin, multipathd will receive fpin notifications, set path states to "marginal" accordingly, and regroup paths as described for on. This option can't be used in combination with other options for "Shaky path detection" (see below). Note: If this is set to fpin, the marginal_path_* and san_path_err_* options are implicitly set to no. Also, this option cannot be switched either to or from fpin on a multipathd reconfigure. multipathd must be restarted for the change to take effect. See "Shaky paths detection" below for more information.

The default is: off

find_multipaths

This option controls whether multipath and multipathd try to create multipath maps over non-blacklisted devices they encounter. This matters a) when a device is encountered by multipath -u during udev rule processing (a device is blocked from further processing by higher layers - such as LVM - if and only if it´s considered a valid multipath device path), and b) when multipathd detects a new device. The following values are possible:

strict

Both multipath and multipathd treat only such devices as multipath devices which have been part of a multipath map previously, and which are therefore listed in the wwids_file. Users can manually set up multipath maps using the multipathd add map command. Once set up manually, the map is remembered in the wwids file and will be set up automatically in the future.

no

Multipath behaves like strict. Multipathd behaves like greedy.

yes

Both multipathd and multipath treat a device as multipath device if the conditions for strict are met, or if at least two non-blacklisted paths with the same WWID have been detected.

greedy

Both multipathd and multipath treat every non-blacklisted device as multipath device path.

smart

This differs from find_multipaths yes only in the way it treats new devices for which only one path has been detected yet. When such a device is first encountered in udev rules, it is treated as a multipath device. multipathd waits whether additional paths with the same WWID appears. If that happens, it sets up a multipath map. If it doesn´t happen until a timeout expires, or if setting up the map fails, a new uevent is triggered for the device; at second encounter in the udev rules, the device will be treated as non-multipath and passed on to upper layers. Note: this may cause delays during device detection if there are single-path devices which aren´t blacklisted.

The default is: strict

find_multipaths_timeout

Timeout, in seconds, to wait for additional paths after detecting the first one, if find_multipaths "smart" (see above) is set. If the value is positive, this timeout is used for all unknown, non-blacklisted devices encountered. If the value is negative (recommended), it's only applied to "known" devices that have an entry in multipath's hardware table, either in the built-in table or in a device section; other ("unknown") devices will use a timeout of only 1 second to avoid booting delays. The value 0 means "use the built-in default". If find_multipath has a value other than smart, this option has no effect.

The default is: -10 (10s for known and 1s for unknown hardware)

uxsock_timeout

CLI receive timeout in milliseconds. For larger systems CLI commands might timeout before the multipathd lock is released and the CLI command can be processed. This will result in errors like "timeout receiving packet" to be returned from CLI commands. In these cases it is recommended to increase the CLI timeout to avoid those issues.

The default is: 4000

retrigger_tries

Sets the number of times multipathd will try to retrigger a uevent to get the WWID.

The default is: 3

retrigger_delay

Sets the amount of time, in seconds, to wait between retriggers.

The default is: 10

missing_uev_wait_timeout

Controls how many seconds multipathd will wait, after a new multipath device is created, to receive a change event from udev for the device, before automatically enabling device reloads. Usually multipathd will delay reloads on a device until it receives a change uevent from the initial table load.

The default is: 30

skip_kpartx

If set to yes , kpartx will not automatically create partitions on the device.

The default is: no

disable_changed_wwids

This option is deprecated and ignored. If the WWID of a path suddenly changes, multipathd handles it as if it was removed and then added again.

remove_retries

This sets how may times multipath will retry removing a device that is in-use. Between each attempt, multipath will sleep 1 second.

The default is: 0

max_sectors_kb

Sets the max_sectors_kb device parameter on all path devices and the multipath device to the specified value.

The default is: in /sys/block/<dev>/queue/max_sectors_kb

ghost_delay

Sets the number of seconds that multipath will wait after creating a device with only ghost paths before marking it ready for use in systemd. This gives the active paths time to appear before the multipath runs the hardware handler to switch the ghost paths to active ones. Setting this to 0 or no makes multipath immediately mark a device with only ghost paths as ready.

The default is: no

enable_foreign

Enables or disables foreign libraries (see section FOREIGN MULTIPATH SUPPORT below). The value is a regular expression; foreign libraries are loaded if their name (e.g. "nvme") matches the expression. By default, no foreign libraries are enabled. Set this to "nvme" to enable NVMe native multipath support, or ".*" to enable all foreign libraries.

The default is: "NONE"

recheck_wwid

If set to yes, when a failed path is restored, its wwid is rechecked. If the wwid has changed, the path is removed from the current multipath device, and re-added as a new path. Multipathd will also recheck a path's wwid if it is manually re-added. This option only works for SCSI devices that are configured to use the default uid_attribute, ID_SERIAL, or sysfs for getting their wwid.

The default is: no

## blacklist and blacklist_exceptions sections
The blacklist section is used to exclude specific devices from the multipath topology. It is most commonly used to exclude local disks or non-disk devices (such as LUNs for the storage array controller) from being handled by multipath-tools.

In the blacklist and blacklist_exceptions sections, starting a quoted value with an exclamation mark "!" will invert the matching of the rest of the regular expression. For instance, "!^sd[a-z]" will match all values that do not start with "sd[a-z]". The exclamation mark can be escaped "\!" to match a literal ! at the start of a regular expression. Note: The exclamation mark must be inside quotes, otherwise it will be treated as starting a comment.

The blacklist_exceptions section is used to revert the actions of the blacklist section. This allows one to selectively include ("whitelist") devices which would normally be excluded via the blacklist section. A common usage is to blacklist "everything" using a catch-all regular expression, and create specific blacklist_exceptions entries for those devices that should be handled by multipath-tools.

The following keywords are recognized in both sections. The defaults are empty unless explicitly stated.

devnode

Regular expression matching the device nodes to be excluded/included.

The default blacklist consists of the regular expression "!^(sd[a-z]|dasd[a-z]|nvme[0-9])". This causes all device types other than scsi, dasd, and nvme to be excluded from multipath handling by default.

wwid

Regular expression for the World Wide Identifier of a device to be excluded/included.

device

Subsection for the device description. This subsection recognizes the vendor and product keywords. Both are regular expressions. For a full description of these keywords please see the devices section description.

property

Regular expression for an udev property. All devices that have matching udev properties will be excluded/included. The handling of the property keyword is special, because devices must have at least one whitelisted udev property; otherwise they're treated as blacklisted, and the message "blacklisted, udev property missing" is displayed in the logs.

Note: The behavior of this option has changed in multipath-tools 0.8.2 compared to previous versions. Blacklisting by missing properties is only applied to devices which do have the property specified by uid_attribute (e.g. ID_SERIAL) set. Previously, it was applied to every device, possibly causing devices to be blacklisted because of temporary I/O error conditions.

The default blacklist exception is: (SCSI_IDENT_|ID_WWN), causing well-behaved SCSI devices and devices that provide a WWN (World Wide Number) to be included, and all others to be excluded.

protocol

Regular expression for the protocol of a device to be excluded/included.

The protocol strings that multipath recognizes are scsi:fcp, scsi:spi, scsi:ssa, scsi:sbp, scsi:srp, scsi:iscsi, scsi:sas, scsi:adt, scsi:ata, scsi:unspec, nvme:pcie, nvme:rdma, nvme:fc, nvme:tcp, nvme:loop, nvme:apple-nvme, nvme:unspec, ccw, cciss, and undef. The protocol that a path is using can be viewed by running multipathd show paths format "%d %P"

For every device, these 5 blacklist criteria are evaluated in the order "property, devnode, device, protocol, wwid". If a device turns out to be blacklisted by any criterion, it's excluded from handling by multipathd, and the later criteria aren't evaluated any more. For each criterion, the whitelist takes precedence over the blacklist if a device matches both.

Note: Besides the blacklist and whitelist, other configuration options such as find_multipaths have an impact on whether or not a given device is handled by multipath-tools.

## multipaths section
The multipaths section allows setting attributes of multipath maps. The attributes that are set via the multipaths section (see list below) take precedence over all other configuration settings, including those from the overrides section.

The only recognized attribute for the multipaths section is the multipath subsection. If there are multiple multipath subsections matching a given WWID, the contents of these sections are merged, and settings from later entries take precedence.

The multipath subsection recognizes the following attributes:

wwid

(Mandatory) World Wide Identifier. Detected multipath maps are matched against this attribute. Note that, unlike the wwid attribute in the blacklist section, this is not a regular expression or a substring; WWIDs must match exactly inside the multipaths section.

alias

Symbolic name for the multipath map. This takes precedence over a an entry for the same WWID in the bindings_file.

The following attributes are optional; if not set the default values are taken from the overrides, devices, or defaults section:

path_grouping_policy

path_selector

prio

prio_args

failback

rr_weight

no_path_retry

rr_min_io

rr_min_io_rq

flush_on_last_del

features

reservation_key

user_friendly_names

deferred_remove

san_path_err_threshold

san_path_err_forget_rate

san_path_err_recovery_time

marginal_path_err_sample_time

marginal_path_err_rate_threshold

marginal_path_err_recheck_gap_time

marginal_path_double_failed_time

delay_watch_checks

delay_wait_checks

skip_kpartx

max_sectors_kb

ghost_delay

## devices section
Important:

The built-in hardware device table of multipath-tools is created by members of the Linux community in the hope that it will be useful. The existence of an entry for a given storage product in the hardware table does not imply that the product vendor supports, or has tested, the product with multipath-tools in any way. Always consult the vendor's official documentation for support-related information.

multipath-tools have a built-in device table with reasonable defaults for more than 100 known multipath-capable storage devices. The devices section can be used to override these settings. If there are multiple matches for a given device, the attributes of all matching entries are applied to it. If an attribute is specified in several matching device subsections, later entries take precedence. Thus, entries in files under config_dir (in reverse alphabetical order) have the highest precedence, followed by entries in multipath.conf; the built-in hardware table has the lowest precedence. Inside a configuration file, later entries have higher precedence than earlier ones.

The only recognized attribute for the devices section is the device subsection. Devices detected in the system are matched against the device entries using the vendor, product, and revision fields, which are all POSIX Extended regular expressions (see regex(7)).

The vendor, product, and revision fields that multipath or multipathd detect for devices in a system depend on the device type. For SCSI devices, they correspond to the respective fields of the SCSI INQUIRY page. In general, the command 'multipathd show paths format "%d %s"' command can be used to see the detected properties for all devices in the system.

The device subsection recognizes the following attributes:

vendor

(Mandatory) Regular expression to match the vendor name.

product

(Mandatory) Regular expression to match the product name.

revision

Regular expression to match the product revision. If not specified, any revision matches.

product_blacklist

Products with the given vendor matching this string are blacklisted. This is equivalent to a device entry in the blacklist section with the vendor attribute set to this entry's vendor, and the product attribute set to the value of product_blacklist.

alias_prefix

The user_friendly_names prefix to use for this device type, instead of the default "mpath".

vpd_vendor

The vendor specific vpd page information, using the vpd page abbreviation. The vpd page abbreviation can be found by running sg_vpd -e. multipathd will use this information to gather device specific information that can be displayed with the %g wildcard for the multipathd show maps format and multipathd show paths format commands. Currently only the hp3par vpd page is supported.

hardware_handler

The hardware handler to use for this device type. The following hardware handler are implemented:

1 emc

(Hardware-dependent) Hardware handler for DGC class arrays as CLARiiON CX/AX and EMC VNX families with Failover Mode 1 (Passive Not Ready(PNR)).

1 rdac

(Hardware-dependent) Hardware handler for LSI/Engenio/NetApp RDAC class as NetApp SANtricity E/EF Series and rebranded arrays, with "Linux DM-MP (Kernel 3.9 or earlier)" option.

1 hp_sw

(Hardware-dependent) Hardware handler for HP/COMPAQ/DEC HSG80 and MSA/HSV arrays with Active/Standby mode exclusively.

1 alua

(Hardware-dependent) Hardware handler for SCSI-3 ALUA compatible arrays.

1 ana

(Hardware-dependent) Hardware handler for NVMe ANA compatible arrays.

The default is: <unset>

Important Note: Linux kernels 4.3 and newer automatically attach a device handler to known devices (which includes all devices supporting SCSI-3 ALUA) and disallow changing the handler afterwards. Setting hardware_handler for such devices on these kernels has no effect.

The following attributes are optional; if not set the default values are taken from the defaults section:

path_grouping_policy

uid_attribute

path_selector

path_checker

prio

prio_args

features

failback

rr_weight

no_path_retry

rr_min_io

rr_min_io_rq

fast_io_fail_tmo

dev_loss_tmo

eh_deadline

flush_on_last_del

user_friendly_names

retain_attached_hw_handler

detect_prio

detect_checker

deferred_remove

san_path_err_threshold

san_path_err_forget_rate

san_path_err_recovery_time

marginal_path_err_sample_time

marginal_path_err_rate_threshold

marginal_path_err_recheck_gap_time

marginal_path_double_failed_time

delay_watch_checks

delay_wait_checks

skip_kpartx

max_sectors_kb

ghost_delay

all_tg_pt

## overrides section
The overrides section recognizes the following optional attributes; if not set the values are taken from the devices or defaults sections:

path_grouping_policy

uid_attribute

path_selector

path_checker

alias_prefix

features

prio

prio_args

failback

rr_weight

no_path_retry

rr_min_io

rr_min_io_rq

flush_on_last_del

fast_io_fail_tmo

dev_loss_tmo

eh_deadline

user_friendly_names

retain_attached_hw_handler

detect_prio

detect_checker

deferred_remove

san_path_err_threshold

san_path_err_forget_rate

san_path_err_recovery_time

marginal_path_err_sample_time

marginal_path_err_rate_threshold

marginal_path_err_recheck_gap_time

marginal_path_double_failed_time

delay_watch_checks

delay_wait_checks

skip_kpartx

max_sectors_kb

ghost_delay

all_tg_pt

The overrides section also recognizes the optional protocol subsection, and can contain multiple protocol subsections. Path devices are matched against the protocol subsection using the mandatory type attribute. Attributes in a matching protocol subsection take precedence over attributes in the rest of the overrides section. If there are multiple matching protocol subsections, later entries take precedence.

protocol subsection

The protocol subsection recognizes the following mandatory attribute:

type

The protocol string of the path device. The possible values are scsi:fcp, scsi:spi, scsi:ssa, scsi:sbp, scsi:srp, scsi:iscsi, scsi:sas, scsi:adt, scsi:ata, scsi:unspec, nvme:pcie, nvme:rdma, nvme:fc, nvme:tcp, nvme:loop, nvme:apple-nvme, nvme:unspec, ccw, cciss, and undef. This is not a regular expression. the path device protocol string must match exactly. The protocol that a path is using can be viewed by running multipathd show paths format "%d %P"

The following attributes are optional; if not set, the default values are taken from the overrides, devices, or defaults section:

fast_io_fail_tmo

dev_loss_tmo

eh_deadline

## WWID generation
Multipath uses a World Wide Identification (WWID) to determine which paths belong to the same device. Each path presenting the same WWID is assumed to point to the same device.

The WWID is generated by four methods (in the order of preference):

uid_attrs

The WWID is derived from udev attributes by matching the device node name; cf uid_attrs above.

uid_attribute

Use the value of the specified udev attribute; cf uid_attribute above.

sysfs

Try to determine the WWID from sysfs attributes. For SCSI devices, this means reading the Vital Product Data (VPD) page "Device Identification" (0x83).

The default settings (using udev and uid_attribute configured from the built-in hardware table) should work fine in most scenarios. Users who want to enable uevent merging must set uid_attrs.

## Shaky paths detection
A common problem in SAN setups is the occurrence of intermittent errors: a path is unreachable, then reachable again for a short time, disappears again, and so forth. This happens typically on unstable interconnects. It is undesirable to switch pathgroups unnecessarily on such frequent, unreliable events. multipathd supports three different methods for detecting this situation and dealing with it. All methods share the same basic mode of operation: If a path is found to be "shaky" or "flipping", and appears to be in healthy status, it is not reinstated (put back to use) immediately. Instead, it is placed in the "delayed" state and watched for some time, and only reinstated if the healthy state appears to be stable. If the marginal_pathgroups option is set, the path will reinstated immediately, but placed in a special pathgroup for marginal paths. Marginal pathgroups will not be used until all other pathgroups have been tried. At the time when the path would normally be reinstated, it will be returned to its normal pathgroup. The logic of determining "shaky" condition, as well as the logic when to reinstate, differs between the three methods.

"delay_checks" failure tracking

This method is deprecated and mapped to the "san_path_err" method. See the delay_watch_checks and delay_wait_checks options above for more information.

"marginal_path" failure tracking

If a second failure event (good->bad transition) occurs within marginal_path_double_failed_time seconds after a failure, high-frequency monitoring is started for the affected path: I/O is sent at a rate of 10 per second. This is done for marginal_path_err_sample_time seconds. During this period, the path is not reinstated. If the rate of errors remains below marginal_path_err_rate_threshold during the monitoring period, the path is reinstated. Otherwise, it is kept in failed state for marginal_path_err_recheck_gap_time, and after that, it is monitored again. For this method, time intervals are measured in seconds.

"san_path_err" failure tracking

multipathd counts path failures for each path. Once the number of failures exceeds the value given by san_path_err_threshold, the path is not reinstated for san_path_err_recovery_time seconds. While counting failures, multipathd "forgets" one past failure every "san_path_err_forget_rate" ticks; thus if errors don't occur more often then once in the forget rate interval, the failure count doesn't increase and the threshold is never reached. Ticks are the time between path checks by multipathd, which is variable and controlled by the polling_interval and max_polling_interval parameters.

"FPIN " failure tracking

Fibre channel fabrics can notify hosts about fabric-level issues such as integrity failures or congestion with so-called Fabric Performance Impact Notifications (FPINs).On receiving the fpin notifications through ELS multipathd will move the affected path and port states to marginal.

This method is deprecated in favor of the "marginal_path" failure tracking method, and only offered for backward compatibility.

See the documentation of the individual options above for details. It is strongly discouraged to use more than one of these methods for any given multipath map, because the two concurrent methods may interact in unpredictable ways. If the "marginal_path" method is active, the "san_path_err" parameters are implicitly set to 0.

## FOREIGN MULTIPATH SUPPORT
multipath and multipathd can load "foreign" libraries to add support for other multipathing technologies besides the Linux device mapper. Currently this support is limited to printing detected information about multipath setup. In topology output, the names of foreign maps are prefixed by the foreign library name in square brackets, as in this example:

```
# multipath -ll
uuid.fedcba98-3579-4567-8765-123456789abc [nvme]:nvme4n9 NVMe,Some NVMe controller,FFFFFFFF
size=167772160 features='n/a' hwhandler='ANA' wp=rw
|-+- policy='n/a' prio=50 status=optimized
| `- 4:38:1    nvme4c38n1 0:0     n/a   optimized    live
`-+- policy='n/a' prio=50 status=optimized

  `- 4:39:1    nvme4c39n1 0:0     n/a   optimized    live
```

The "nvme" foreign library provides support for NVMe native multipathing in the kernel. It is part of the standard multipath package.

## KNOWN ISSUES
The usage of queue_if_no_path option can lead to D state processes being hung and not killable in situations where all the paths to the LUN go offline. It is advisable to use the no_path_retry option instead.

The use of queue_if_no_path or no_path_retry might lead to a deadlock if the dev_loss_tmo setting results in a device being removed while I/O is still queued. The multipath daemon will update the dev_loss_tmo setting accordingly to avoid this deadlock. Hence if both values are specified the order of precedence is no_path_retry, queue_if_no_path, dev_loss_tmo.

## SEE ALSO
udev(8), dmsetup(8), multipath(8), multipathd(8).

## AUTHORS
multipath-tools was developed by Christophe Varoqui, <christophe.varoqui@opensvc.com> and others.


mbox

package: mutt

title: mbox(5)

summary: Format for mail message storage.

text:
# mbox(5)

## NAME
mbox - Format for mail message storage.

## DESCRIPTION
This document describes the format traditionally used by Unix hosts to store mail messages locally. mbox files typically reside in the system's mail spool, under various names in users' Mail directories, and under the name mbox in users' home directories.

An mbox is a text file containing an arbitrary number of e-mail messages. Each message consists of a postmark, followed by an e-mail message formatted according to RFC822, RFC2822. The file format is line-oriented. Lines are separated by line feed characters (ASCII 10).

A postmark line consists of the four characters "From", followed by a space character, followed by the message's envelope sender address, followed by whitespace, and followed by a time stamp. This line is often called From_ line.

The sender address is expected to be addr-spec as defined in RFC2822 3.4.1. The date is expected to be date-time as output by asctime(3). For compatibility reasons with legacy software, two-digit years greater than or equal to 70 should be interpreted as the years 1970+, while two-digit years less than 70 should be interpreted as the years 2000-2069. Software reading files in this format should also be prepared to accept non-numeric timezone information such as "CET DST" for Central European Time, daylight saving time.

Example:

>From example@example.com Fri Jun 23 02:56:55 2000

In order to avoid misinterpretation of lines in message bodies which begin with the four characters "From", followed by a space character, the mail delivery agent must quote any occurrence of "From " at the start of a body line.

There are two different quoting schemes, the first (MBOXO) only quotes plain "From " lines in the body by prepending a '>' to the line; the second (MBOXRD) also quotes already quoted "From " lines by prepending a '>' (i.e. ">From ", ">>From ", ...). The later has the advantage that lines like

>From the command line you can use the '-p' option

aren't dequoted wrongly as a MBOXRD-MDA would turn the line into

>>From the command line you can use the '-p' option

before storing it. Besides MBOXO and MBOXRD there is also MBOXCL which is MBOXO with a "Content-Length:"-field with the number of bytes in the message body; some MUAs (like mutt(1)) do automatically transform MBOXO mailboxes into MBOXCL ones when ever they write them back as MBOXCL can be read by any MBOXO-MUA without any problems.

If the modification-time (usually determined via stat(2)) of a nonempty mbox file is greater than the access-time the file has new mail. Many MUAs place a Status: header in each message to indicate which messages have already been read.

## LOCKING
Since mbox files are frequently accessed by multiple programs in parallel, mbox files should generally not be accessed without locking.

Three different locking mechanisms (and combinations thereof) are in general use:

- fcntl(2) locking is mostly used on recent, POSIX-compliant systems. Use of this locking method is, in particular, advisable if mbox files are accessed through the Network File System (NFS), since it seems the only way to reliably invalidate NFS clients' caches.

- flock(2) locking is mostly used on BSD-based systems.

- Dotlocking is used on all kinds of systems. In order to lock an mbox file named folder, an application first creates a temporary file with a unique name in the directory in which the folder resides. The application then tries to use the link(2) system call to create a hard link named folder.lock to the temporary file. The success of the link(2) system call should be additionally verified using stat(2) calls. If the link has succeeded, the mail folder is considered dotlocked. The temporary file can then safely be unlinked.

In order to release the lock, an application just unlinks the folder.lock file.

If multiple methods are combined, implementors should make sure to use the non-blocking variants of the fcntl(2) and flock(2) system calls in order to avoid deadlocks.

If multiple methods are combined, an mbox file must not be considered to have been successfully locked before all individual locks were obtained. When one of the individual locking methods fails, an application should release all locks it acquired successfully, and restart the entire locking procedure from the beginning, after a suitable delay.

The locking mechanism used on a particular system is a matter of local policy, and should be consistently used by all applications installed on the system which access mbox files. Failure to do so may result in loss of e-mail data, and in corrupted mbox files.

## FILES
/var/spool/mail/$LOGNAME

$LOGNAME's incoming mail folder.

$HOME/mbox

user's archived mail messages, in his $HOME directory.

$HOME/Mail/

A directory in user's $HOME directory which is commonly used to hold mbox format folders.

## SEE ALSO
mutt(1), fcntl(2), flock(2), link(2), stat(2), asctime(3), maildir(5), mmdf(5), RFC822, RFC976, RFC2822

## AUTHOR
Thomas Roessler <roessler@does-not-exist.org>, Urs Janssen <urs@tin.org>

## HISTORY
The mbox format occurred in Version 6 AT&T Unix.

A variant of this format was documented in RFC976.


mmdf

title: mmdf(5)

summary: Multi\-channel Memorandum Distribution Facility mailbox format

text:
# mmdf(5)

## NAME
MMDF - Multi-channel Memorandum Distribution Facility mailbox format

## DESCRIPTION
This document describes the MMDF mailbox format used by some MTAs and MUAs (i.e. scomail(1)) to store mail messages locally.

An MMDF mailbox is a text file containing an arbitrary number of e-mail messages. Each message consists of a postmark, followed by an e-mail message formatted according to RFC822 / RFC2822, followed by a postmark. The file format is line-oriented. Lines are separated by line feed characters (ASCII 10). A postmark line consists of the four characters "^A^A^A^A" (Control-A; ASCII 1).

Example of a MMDF mailbox holding two mails:

```
^A^A^A^A

From: example@example.com

To: example@example.org

Subject: test

>From what I learned about the MMDF-format:

^A^A^A^A

^A^A^A^A

From: example@example.com

To: example@example.org

Subject: test 2

bar

^A^A^A^A
```

In contrast to most other single file mailbox formats like MBOXO and MBOXRD (see mbox(5)) there is no need to quote/dequote "From "-lines in MMDF mailboxes as such lines have no special meaning in this format.

If the modification-time (usually determined via stat(2)) of a nonempty mailbox file is greater than the access-time the file has new mail. Many MUAs place a Status: header in each message to indicate which messages have already been read.

## LOCKING
Since MMDF files are frequently accessed by multiple programs in parallel, MMDF files should generally not be accessed without locking.

Three different locking mechanisms (and combinations thereof) are in general use:

- fcntl(2) locking is mostly used on recent, POSIX-compliant systems. Use of this locking method is, in particular, advisable if MMDF files are accessed through the Network File System (NFS), since it seems the only way to reliably invalidate NFS clients' caches.

- flock(2) locking is mostly used on BSD-based systems.

- Dotlocking is used on all kinds of systems. In order to lock an MMDF file named folder, an application first creates a temporary file with a unique name in the directory in which the folder resides. The application then tries to use the link(2) system call to create a hard link named folder.lock to the temporary file. The success of the link(2) system call should be additionally verified using stat(2) calls. If the link has succeeded, the mail folder is considered dotlocked. The temporary file can then safely be unlinked.

In order to release the lock, an application just unlinks the folder.lock file.

If multiple methods are combined, implementors should make sure to use the non-blocking variants of the fcntl(2) and flock(2) system calls in order to avoid deadlocks.

If multiple methods are combined, an MMDF file must not be considered to have been successfully locked before all individual locks were obtained. When one of the individual locking methods fails, an application should release all locks it acquired successfully, and restart the entire locking procedure from the beginning, after a suitable delay.

The locking mechanism used on a particular system is a matter of local policy, and should be consistently used by all applications installed on the system which access MMDF files. Failure to do so may result in loss of e-mail data, and in corrupted MMDF files.

## CONFORMING TO
MMDF is not part of any currently supported standard.

## HISTORY
MMDF was developed at the University of Delaware by Dave Crocker.

## SEE ALSO
scomail(1), fcntl(2), flock(2), link(2), stat(2), mbox(5), RFC822, RFC2822

## AUTHOR
Urs Janssen <urs@tin.org>